There's a recipe that turns coal dust to ash, but it doesn't unlock until much later.

Before you get diamond processing unlocked (presuming you're running the full or full-ish suite) your options are basically smelt/boiler fuel, silicon, charcoal briquettes, and tossing them into warehouses. None are particularly great options:

smelter/boiler fuel:

steam engines usually lose out to combustion turbines, and for it to be reliable, it needs a steady stream of another fuel type
the highest volume smelting processes get forced over to coke fairly early in the rawore processing stages

silicon:

mostly delays the problem, as the use for it won't kick in until you're working on circuit 2 ingredients, and that will end up bottlenecked on other things

briquettes:

you can't use them for anything but raw boiler/smelt fuel until much much later, and the glycerol to make them will need a fair amount of nichrome to make oleochemicals

warehouses:

it will fill up eventually

The good news is that once you unlock the coal dust -> kimberlite recipe with the basic diamond processing tech, that can serve as an effectively bottomless pit